This invention relates generally to television high voltage systems and particularly to shutdown circuits used therein.
In a typical color television receiver scansion and display system, a cathode ray tube (CRT) display device includes a trio of electron beam sources which are directed at a tri-color phosphor viewing screen. Horizontal and vertical scansion circuitry within the receiver locally generate scansion signals which are synchronized to reference information within the received signal. The scansion signals are applied to an electromagnetic deflection yoke positioned on the envelope of the CRT producing vertical and horizontal scansion of the viewing screen. The cathode ray tube requires a number of operating potentials the highest of which is an accelerating potential of approximately 25 to 30 kilovolts which is generally referred to as the high voltage. This potential is applied to an electrode within the CRT to accelerate the electrons within the directed beams to an energy level sufficient to cause light emission by impacted phosphor areas and illumination of the viewing screen.
In the great majority of television receivers the horizontal scansion system produces this high voltage in addition to the horizontal scansion signals. Horizontal scansion includes a relatively slow scan deflection of the electron beams followed by a relatively fast retrace deflection in which the beams are deflected back to the "start" of scan position. The scansion signal producing this retract comprises a short duration high amplitude pulse which is also used to generate CRT high voltage. Most receivers use a tertiary winding on the horizontal transformer together with a rectifier or voltage multiplier to raise the voltage to sufficient level for high voltage production. In the former system the tertiary transformer winding ouput is rectified directly while in the latter a familiar capacitor diode matrix is used to boost and rectify the voltage.
It is generally desirable for purposes of picture sharpness, brightness and color rendition to maintain a relatively high accelerating potential. However, cathode ray tubes have a tendency to produce prohibitive amounts of radiation when excessive high voltage is used. As a result care must be taken to assure that the accelerating potential does not exceed that radiation producing threshold.
The problems of radiation production in modern television receiver cathode ray tubes are made more difficult by the improved regulation of operating supply and high voltage generating circuitry. Modern scansion circuitry is able to sustain great overloads and still maintain high voltage ouput. Because these improved systems are capable of producing greater power levels they are not in any real sense "self-limiting". Therefore, while providing considerable advantages in picture quality and other performance criteria, such "stronger" high voltage supplies also have a greater capacity for continued operation even though high voltage has somehow exceeded the radiation producing threshold.
It is well known to employ high voltage shutdown circuitry which monitors the accelerating potential or some related voltage and disables the accelerating potential circuitry in the event of excess output. Such systems may include threshold circuitry detecting either the high voltage itself or a voltage derived therefrom such as that used for CRT focus. Another alternative uses circuitry detecting the amplitude of the retrace portion of the horizontal scansion signal which, of course, varies in a predictable relationship with CRT high voltage. In any case, the most typical operation provides complete shutdown of the high voltage system in the event of an excess.
Because such shutdown circuits may be required to respond to relatively small high voltage changes, it is desirable to maximize system sensitivity. However, the system must also be relatively insensitive to false triggering caused, for example, by CRT arcing, transitory noise as well as variations due to temperature drifts and components aging within the receiver circuitry. These two conflicting criteria may be reconciled to the extent that the portion of high voltage changes available to the system is maximized. It is desirable, therefore, to maximize the conversion efficiency of the high voltage detector in a shutdown circuit.